This invention relates to transient protection circuits, and more particularly, to a transient protection circuit for clamping an applied voltage to a predetermined value while suppressing surge currents.
The application of transient protection circuits is well understood in electronic systems especially for those exposed to potentially adverse external conditions. In the case of telephone communication systems, for example, the electronic components throughout the system are susceptible to high voltage transients appearing along the transmission lines, possibly caused by nearby lightning strikes which may induce a high intensity electromagnetic field about the transmission line conductors, or inadvertent contact with high voltage power lines. The high voltage transients can induce surge currents which may exceed the operating rating of the electronic components within the system, causing undue stress if not permanent damage thereto.
In the telephone system example, the central telephone office and the end user, for example a residential phone site, are usually connected via a communication loop comprising two transmission lines, commonly referred to as TIP and RING, nominally energized to 48 volts DC and modulated with an AC signal for the voice data. The electronic circuitry coupled for sending and receiving the communication signals over the transmission lines are typically designed for a peak operating range of 150 volts and therefore must be protected from the high voltage transients. Accordingly, a primary voltage protection circuit is often installed at the entrance of the central telephone office for clamping the potential developed across the transmission lines to say 1000 volts. If lightning should strike nearby creating a high voltage transient on the telephone lines, the primary voltage protection circuit clamps the applied voltage to 1000 volts, while a secondary protection circuit further limits the voltage applied to the sensitive electronic components. The nature of the primary and secondary protection is more thoroughly described in the detailed description of the prior art included herein; however, in general conventional transient protection circuits are voltage clamping devices and do not inhibit the surge current induced by the electromagnetic field. Thus, it is still possible for significant currents to flow through the telephone systems during such high voltage transients in spite of the voltage clamping protection, possibly causing damage to the telephone switching equipment.
Hence, there is a need for a transient protection circuit for suppressing the surge current induced by the high voltage transient conditions in addition to providing the voltage clamping thereof.